Self-balancing centrifuge method and apparatus

ABSTRACT

A centrifuge of the type used for a number of individual test samples in separate sample containers is dynamically balanced automatically by means of a counterbalancing mass contained in an annular chamber within the centrifuge wheel. The centrifuge wheel is mounted as an inverted pendulum about a fixed pivot and spring biased to the vertical position. The counterbalancing mass, which is at the same vertical level as the sample containers, automatically moves angularly to the counterbalancing position at speeds above the speed corresponding to the natural vibration frequency of the pendulum system.

United States Patent v Livshitz et al.

[ 51 Sept. 19, 1972 [54] SELF-BALANCING CENTRIFUGE METHOD AND APPARATUSInventors: Stanley L. Livshitz, Bronx, N.Y.;

Robert Alan Mayo, Verona, NJ.

Assignee: Technicon Instruments Corporation,

Tarrytown, NY.

Filed: Oct. 30, 1970 Appl. No.: 85,348

US. Cl ..233/26, 74/573 Int. Cl. ..B04b 9/12 Field of Search.....233l23R, 26, l R, 1 C, 23 A;

References Cited UNITED STATES PATENTS 5/1924 Swancara ..233/26 1/1968Jarvis ..210/363 9/1959 Gibson ..21'0/363 I 1/1892 Berg ..233/261,760,775 5/1930 Peters ..2l0/363 FOREIGN PATENTS OR APPLICATIONS1,091,525 10/1960 Germany ..2l0/363 Primary Examiner-Jordan FranklinAssistant Examiner-George M. Krizmanich Attorney-S. P. Tedesco and S. E.Rockwell [57] ABSTRACT A centrifuge of the type used for a number ofindividual test samples in separate sample containers is dynamicallybalanced automatically by means of a counterbalancing mass contained inan annular chamber within the centrifuge wheel. The centrifuge wheel ismounted as an inverted pendulum about a fixed pivot and spring biased tothe vertical position. The counterbalancing mass, which is at the samevertical level as the sample containers, automatically moves angularlyto the counter-balancing position at speeds above the speedcorresponding to the natural vibration frequency of the pendulum system.

18 Claims, 2 Drawing Figures I0 I II lllll hm H h l t 2O 0; I, I\ itINVENTOR.

ROBERT A. MAYO STANLEY L. LIVSHITZ PATENTEUSEP 1 9 m SHEET 2 OF 2INYENTOR. ROBERT A. MAYO STANLEY L. LIVSHITZ BY SELF-BALANCINGCENTRIFUGE METHOD AND APPARATUS BACKGROUND OF THE INVENTION 1 Field ofthe Invention This invention relates to improved centrifuges of the typewhich are particularly useful for separating small sample quantities ofchemical or biological fluids such as blood samples, the samples beingcontained in a plurality of small tubes which are carried at theperiphery of a centrifuge wheel.

2. Description of the Prior Art One of the most serious problems in theoperation of centrifuges of the above type is the problem of imbalanceof the centrifuge due to unbalanced loading. There are two basic causesfor the unbalanced loading conditions. A minor one of these causes isthe factor that the individual containers of liquid to be separated bythe centrifuge are not necessarily loaded equally with fluid. However,the major cause of unbalance is a failure to fully load the centrifuge,some of the cups or carriers of the centrifuge being allowed to remainempty while the others are filled.

The presently available solutions for these problems of imbalance havenot been particularly satisfactory. For instance, it is not alwayspractical to solve the imbalance problem by waiting until the centrifugeis completely filled before running it because there are often urgenttests which must be made immediately without waiting for subsequentsamples to be collected. Another obvious solution is to load thecentrifuge symmetrically, evenly spacing the full cups and empty cupsaround the peripheral edge of the centrifuge, or to fill all empty cupswith dummy samples of water. This procedure has a number of importantdisadvantages. For one, it consumes extra operator time. Furthermore, itis extremely important to-maintain absolutely accurate identification ofindividual samples. One of the means for accomplishing this is to keepthe samples in absolutely strict order and this objective is most easilyaccomplished by loading the samples into the centrifuge cups in anuninterrupted order without empty cups intervening between adjacentsamples. Still further, it has been proposed in accordance with theteachings of a prior related co-pending patent application hat acentrifuge of the above described type should be employed in conjunctionwith an automatic sampling apparatus, and for this purpose the samplesmust be loaded in a continuous series in adjacent sample cups withoutinterruption. The prior related patentapplication is application Ser.No. 70,199 filed on Sept. 8, 1970 by Stanley L. Livshitz, AlvinEngelhardt, and Stanford L. Adler for a New and Improved SampleProcessing and Supply Appraatus and assigned to the same assignee as thepresent application.

OBJECTS OF THE INVENTION Accordingly, it is an important object of thepresent invention to provide an improved centrifuge method and apparatuswhereby the centrifuge is self-balancing and which will thusautomatically compensate for unbalanced conditions.

Other objects and advantages of the invention will be apparent from thefollowing description and the accompanying drawings.

SUMMARY OF THE INVENTION The present invention may be carried out forbalancing a centrifuge of the type having a plurality of separate cupsarranged around the periphery of the centrifuge wheel for the purpose ofholding containers of liquids to be centrifuged by providing an annularchamber within the wheel, pivotally mounting the centrifuge at its axisas a pendulum, spring biasing the centrifuge to a substantially verticalaxis position, placing at least two balancing weight bodies looselywithin the annular chamber for free movement therein, and then operatingthe centrifuge at a speed substantially in excess of that correspondingto the natural frequency of the spring biased pendulum system to causethe balancing weight bodies to move within the annular chamber topositions compensating for any unbalanced condition in the loading ofthe centrifuge cups.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partially in section,illustrating a preferred embodiment of the invention.

FIG. 2 is a partial schematic sectional top view, taken at section 2-2of FIG. 1, and serving to illustrate features of the dynamic operationof the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring particularly to FIG. 1,there is shown a centrifuge wheel 10 including a removable upper wheelportion 12, sometimes referred to hereinafter as a tray arranged forholding a number of sample tubes 14 of liquid to be separated bycentrifugal action. The wheel 10 also includes a lower, non-removable,portion 16 which is attached for rotation upon a shaft 18. Shaft 18 isan extension of the shaft of a drive motor 20 by means of which thewheel 10 is rotated.

All of the components thus far described are mounted for support on aplate 22 which is pivotally anchored by means of a ball pivot structure24 to a base supporting member 26. The base member 26 may comprise apart of a larger structure, or it may be securely fastened to a rigidfloor. The mounting plate 22 is spring biased to a substantiallyhorizontal position by means of a series of springs 28 spaced around theperipheral edges of the plate. By means of this spring biasing, theshaft 18 of the centrifuge is spring biased to a substantially verticalposition. The pivot 24 is concentric with the center line of theshaft18. The pivotal mounting of the centrifuge, with its spring bias to thevertical position, may be appropriately described as an invertedpendulum mounting and the pendulum possesses a natural oscillation or avibration frequency about the pivot 24 which is determined by thephysical dimensions and characteristics of the system, and particularlyby the mass of the wheel 10 and the spring rate of the springs 28.

The lower portion 16 of the wheel 10 defines an annular chamber 30containing a counterweight mass 32 which automatically shifts within thecavity to counterbalance for any unbalance in the loading of the sampletubes 14. This automatic counterbalancing operation will be described inmore detail below.

The tray portion 12 of the wheel 10 includes holders 34 and 34A for thesample tubes 14, and the holders 34 and 34A are pivotally mounted, as at36, to the peripheral edges of the tray wheel portion 12, so that theymay freely swing radially outwardly when the centrifuge wheel isspinning.

As previously mentioned above, the individual sample containers 14 maybe filled to various levels, and individual tube holders 34 may remainempty. These two factors may cause the centrifuge to be rather seriouslyunbalanced. This causes the centrifuge to wobble as it rotates so thatthe shaft 18 defines a rotating cone of revolution. Thecenter of thepivotal motion is the ball pivot structure 24. The wobbling motion isresisted by the springs 28. The wobbling motion may be referred tohereinafter as rotational vibration, or simply as vibration. Theunbalanced condition is illustrated in FIG. 1 by the presence of asample tube 14 within the holder 34, and the absence of a sample tubewithin the holder 34A. At speeds below that corresponding to the naturalvibration frequency of the system, the counterbalancing mass 32 tends tomove within the cavity 30 to the unbalanced side of the centrifugalwheel so that it does not accomplish a compensating function. However,as soon as the speed of rotation of the centrifuge wheel exceeds thenatural frequency of the pendulum system, there is a phase reversal ofthe forces upon the compensating mass 32 and it is caused to shiftwithin the cavity 30 towards the lightly loaded side of the centrifugewheel to compensate for the unbalanced forces.

The principle of the phase reversal and the automatic positioning of thecompensating mass 32 in an inverted pendulum system of this type isexplained more fully at pages 6-25, 6-26 of the Shock and VibrationHandbook, Volume I, published by McGraw Hill in 1961. While thecompensating mass 32 may consist of a liquid, in the preferred form ofthe invention it consists of at least two solid bodies which are capableof sliding or rolling within the cavity 30 to accomplish thecompensation effect.

Auxiliary buffer springs 42 may be provided, if desired, to prevent thewobbling excursion from becoming too great for speeds at and below thatcorresponding to the natural vibration frequency in the case of a veryserious unbalanced condition. This prevents damage to the apparatus. Asanother alternative feature, the centrifuge of the present invention mayalso include a hydraulic energy absorption device 38 having a pistonplunger 40. Device 38 provides energy absorption to prevent a drasticbuild-up of vibration as the natural resonant frequency of the system ofthe inverted pendulum is approached. The energy absorber 38 may be ofthe hydraulic fluid type in which fluid is forced through an orifice.The device 38 may be rigidly mounted by means of a threaded boss into asuitable threaded hole within the plate 22. The plunger 40 may simply beprovided with a spherical bottom surface which rests against the base 26and is normally biased onto contact therewith by an internal spring. Anumber of the energy absorber devices 38 may be provided and may beinterspersed between adjacent springs 28 around the periphery of theplate 7. In

one preferred embodiment, three springs 28 and three energy absorbers 28were evenly spaced around the periphery of plate 22. The energy absorber38 may be a small hydraulic shock absorber of the type which iscommercially available, for instance, under model number designationshaving the prefix SI-I" from the Special Products Division of IntegratedDynamics Inc. of 2206 Elmwood Avenue, Buffalo, New York 14216. Othersimilar hydraulic damper structures may be employed such as rotarydampers which may be coupled between plate 22 and base members 26 bymeans of rotary operating arms and connecting rods. It is also possibleto provide for some energy absorption by substituting rubber springs forthe springs 28 since the rubber absorbs additional energy as it isdeformed. I

FIG. 2 is a partial schematic sctional top view taken at section 2-2 inFIG. 1. As illustrated in FIG. 2, the mass 32 preferably consists of atleast two solid bodies 32A and 32B,which are preferably cylinders orspheres having a circular profile when viewed from above. These bodiesare rotatably slidable to compensating positions such as illustrated at32A and 328. The cylindrical form of the bodies 32A and 32B is preferredbecause the slight frictional restraint caused by the requirement thatthe cylinders must slide on the bottom Y surface of the chamber 30provides somewhat greater stability at low speeds and provides quieteroperation. At the higher speeds where compensation is obtained, thecentrifugal forces upon the bodies 32A and 32B are so much greater thanthe downward gravity force that the sliding friction is not a seriousrestraining force. It has been determined that the frictional restraintof the cylinders on the bottom surface of the chamber 30 is oftensufficient to keep the cylinders in the dynamic balance position whenthe centrifuge is turned off and slowed down at the end of an operatingcycle. Thus the centrifuge remains balanced as it slows through thespeed corresponding to the natural vibration frequency of the system.The frictional resistance of the cylinders on the bottom surface of thechamber is also advantageous in restraining movement of the cylinders tothe unbalanced side of the centrifuge during the acceleration phase ofthe operating cycle. It is within the scope of the invention to enhancethis feature of the operation of the device by providing non-skidsurfaces on the bottom surface of the changer, or the bottom surface ofthe cylinders, or both. This non-skid feature may be implemented in manyways, including, for instance, roughened surfaces which provide aninterlock effect.

While the counterbalancing mass 32 is referred to generically in FIG. 1,it is represented asa cylinder, such as the cylindrical body 32A, sincethat is the preferred form for the mass. However, it must be emphasizedthat when solid bodies are employed, it is much preferred to use atleast two such bodies so that symmetrical movements of the bodies aroundthe chamber 30 are possible to maintain symmetrical side to side balancewith respect to the point of imbalance of the centrifuge wheel. Thus, asthe bodies 32A and 32B move from the position shown in FIG. 2 to theposition indicated at 32A and 328', they move in a coordinated manner onboth sides of the chamber 30 until a balanced position is achieved. Whencompensating for a maximum unbalanced condition, the bodies wouldproceed to the extreme position opposite to the original position untilthey touched in the bottom most position in FIG. 2. In the event thecentrifuge wheel is actually balanced (independent of bodies 32A and323), then the bodies 32A and 328 end up in positions diametricallyopposite to one another. For any intermediate unbalanced condition, thebodies are positioned at some intermediate position such as shown in thedrawing at 32A and 32B.

While only two of the counterbalancing bodies 32A and 32B areillustrated in the drawings, it will be understood that three or four ormore of these bodies may be employed. However, it is preferably not tofill the cavity 30 more than half full of compensating bodies since thebodies must shift in order to provide a counterbalancing action and ifthe cavity is more than half full, a maximum compensating action is notachievable through the shifting operation. While large bodies whichsubstantially fill the cross-sectional area of the cavity areillustrated, it is within the scope of this invention to employ smallerbodies which nest together and which may be two or more deep across thetoroidal chamber 30.

It is a preferred feature in carrying out the present invention that theupper tray portion 12 of the wheel is easily removable from theremainder of the centrifuge by simply lifting it vertically upward bymeans of the central handle which is provided on the tray 12. Thisprovides quick removal of the tray and all of the loaded cups 34 andquick transfer of the entire tray and all the samples for a combinationwith automatic sample analysis apparatus. This particular featurerelates to a portion of the subject matter described and claimed in theprior copending patent application Ser. No. 70,199 previously mentionedabove.

One important feature of the invention is that the balancing mass 32 issupported within the chamber 30 in a vertical position which is insubstantial horizontal alignment with the unbalanced loadsof the cups34. While this perfect horizontal alignment is not entirely apparentfrom the static picture given by FIG. 1, when the centrifuge is inoperation, the cups 34, whether they are filled or empty, swing radiallyoutwardly about the pivots 36 so that the center of gravity of each cup34 is effectively raised to be substantially horizontally in line withthe mass 32. This feature is important because it provides a moreperfect balancing condition when the ultimate balancing is achieved, acondition in which force couples are absent. If the counterbalancingmass 32 had been located at a different elevation, force couples wouldbe generated in the shaft 18 which would necessarily have to be resistedby the pivot 24 and would tend to cause flexing of the shaft 18. Thesecouples can be very detrimental to the apparatus, causing vibration andexcessive wear of the motor bearings and the spherical pivot 24, andnecessitating heavier construction of the shaft 18.

Because of the perfection of the dynamic balance achievable by means ofthe present invention, it is possible to provide for satisfactoryoperation of the apparatus without the use of the fixed pivot 24. Insuch an arrangement, the entire vertical support of the apparatus isprovided through the springs 28 and the energy absorption devices 38. Insuch a modification, a pivoting or rocking action of the plate 22 isachieved about a pivot point in response to unbalanced conditions.However, the fixed pivot construction employing the spherical ball pointpivot 24 is preferred as it provides vertical stability for theapparatus. It will be understood that other universal pivot structuresmay be employed as alternatives to the spherical bearing shown.

There are a number of important advantages to the pivoted pendulumarrangement of the centrifuge of the present invention. For one, as thecentrifuge begins to spin, the initial deflections of the shaft 18 fromthe vertical position are resisted not only by the springs 28, but alsoby the gyroscopic forces of the spinning wheel 10. Thus, there is agyroscopic stabilization effect since the wobbling motion involves atilting of the axis of rotation.

Furthermore, there is an important advantage to the so-called inverted"pendulum configuration in which the entire centrifuge structure issupported above the pendulum pivot rather than below the pivot. Thisadvantage lies in the fact that the gravity forces acting upon the wheel10 and the shaft 18 combination, whenever the structure is tilted awayfrom the vertical, tend to increase the apparent mass of the pendulumand thus lower the natural vibration frequency of the pendulum. With theaid of this factor, and by selecting an appropriate ratio between themasses of the centrifuge and the spring rate of the springs 28, thenatural frequency of the device is caused to occur at a relatively lowrotational speed. Thus, most of the speed range of the device occursabove the speed corresponding to the natural frequency of the system andcompensation is accordingly available for most of the acceleration,running, and deceleration cycle of the device.

This is an important advantage of the invention because it means thatdynamic balance conditions are approached or achieved before the speedof the device becomes great enough to develop substantial centrifugalforces. In the preferred form of the invention, the speed correspondingto the resonant vibration frequency is less than ten percent of the fulloperating speed of the device. In an actual physical embodiment, forinstance, the speed corresponding to the resonant vibration frequency isabout revolutions per minute, and the full operating speed is at least1,500 revolutions per minute, and more typically about 2,250 revolutionsper minute, or higher.

While we have shown and described the preferred embodiment of ourinvention, it will be understood that the invention may be embodiedotherwise than as herein specifically illustrated or described, and thatcertain changes in the form and arrangement of parts and in the specificmanner of practicing the invention may be made without departing fromthe underlying idea or principles of this invention within the scope ofthe appended claims.

We claim:

1. A method of balancing a centrifuge of the type having a plurality ofseparate cups arranged around the periphery of a centrifuge wheel forthe purpose of holding containers of liquids to be centrifugedcomprising providing an annular chamber within the wheel,

pivotally mounting the centrifuge at its axis as a pendulum,

spring biasing the centrifuge to a substantially vertical axis position,

placing at least two balancing weight bodies loosely within said annularchamber for free movement therein,

7 and then rotating the centrifuge at a speed in excess of thatcorresponding to the natural frequency of the spring biased pendulumsystem to cause the balancing weight bodies to move within the annuteredat said pivot point.

9. A centrifu e as clai ed in clai 4 wherein the combina on of szii dshaft an said wheel is supported as an inverted pendulum with said pivotlar chamber to positions compensating for any un- 5 point positionedbeqeath sfaid f balanced condition in the loading of the centrifuge Acenmfuge as .clalmed m claim 4 when! cups, the balancing weight bodiesbeing positioned sald naimral vlbratlon. frequency corresponds to 8within the annular chamber at substantially the centrifuge speed whichless than ten pelicem of same level as the centrifuge cups during therotathe fun operaimg sPeed the i tion ofthe centrifuge lo 11. Acentrifuge as claimed in claim 8 wherein said supporting means comprisesa fixed supporting base mounting and supporting said ball swivelbearing,

a mounting plate axially spaced away from said sup- 2. The method asclaimed in claim 1 wherein the centrifuge is mounted as an invertedpendulum.

3. A method as claimed in claim 1 wherein porting base,

the force i the spimgs biasmg i c.enmfuge to a said mounting plate beingfixed with respect to the substantially vertical axis position isselected to 15 provide a natural vibration fre uenc of thecenaxialposm-on sa-ld shaft 3 y d h and said spring biasing meanscomprising compresi correspon mg to a can 1 uge spec w sion springsarranged between said supporting is less than ten percent of the normalfull operating base member and said plate and spring biasing said speedofthe cfmmfugei plate to a position substantially parallel to said 4. Aself-balancing centrifuge comprising a rotatable base member shaft 12. Acentrifuge as claimed in claim 1 1 wherein mezfns for suppomng saldshaft permfttmg said drive motor is mounted upon said mounting pivotaldisplacement thereof about a pivot point at plate with its shaftextending vertically upward on F center f of sad fl an axis passingthrough said pivot point,

Sald Suppomng meanP f Spnng basmg said rotatable shaft being connectedto and forming mealls arranged to fi 531d f' an extension of the shaftof said drive motor and stantially vertical position aboutsald pivotpoint, being supported upon and directly driven thereby a motorconnected to mate Said Shaft 13. A centrifuge as claimed in claim 4,wherein a ceflmfuge Wheel mounted P Said shaft at an said centrifugewheel comprises a light weight ax al position thereon substantiallydisplaced from removable sample; tray including the i h l Said P Pportions of said wheel and the cups for receiving Said centrifuge Wheelincluding a plurality of P sample containers and a portion defining saidan- P y connected to the P p y of Said Wheel nular chamber, the sampletray portion of said for mceiving sample containers of liquid t0 bewheel being removable from said annular chamber separated y centrifugeaction, portion for the purpose of transferring the sample said wheeldefining an annular chamber arranged containers as a group forprocessing in another apconcentrically about said shaft, paratus,

a counterbalance mass partially filling said annular 14. A centrifuge asclaimed in claim 4 wherein chamber, there is provided energy absorptionmeans responsaid counterbalance mass being arranged to shift in sive todisplacement movements of said shaft away its angular position withinsaid annular chamber to from the vertical position to absorb thevibration automatically balance said wheel for unequal loadenergy of thecombination of said shaft and said ing of said cups at speeds above thenatural vibrawheel to thereby limit the vibration amplitudes at tionfrequency of the combination of said shaft and low frequencies. saidwheel about said pivot point, said annular 15. Acentrifuge as claimed inclaim 14 wherein chamber being formed to allow the shift of said saidenergy absorption means comprises a plurality counterbalance mass to aposition which is at subof hydraulic shock absorber devices. stantiallythe same level as said cups during rota- A Centrifuge as claimed inclaim 4 wherein i f id centrifuge h said spring biasing means includessnubber springs 5 A centrifuge as l i d i l i 4 h i operable when thevibration excursion exceeds a said counterbalance mass comprises aplurality of p g t h g if g lf fia spring Solid i rate an o t ere y imitu er v1 ration am- 6. A centrifuge as claimed in claim5 wherein Plitudeexcursion Prtwent damage {0 the P' each of said solid bodies has acircular profile when P viewed f above. 17. A centrifuge as claimed inclaim wherein 7 A t if as claimed in claim 4 wherein the configurationof each of said solid bodies 18 such said supporting means includesmeans for maintainas to q f slldmg ent Upon the lower suring asubstantially fixed vertical position for said of Said chamber for theshlft Posmon to shaft while permitting restrained pivotal defiec achlevetions thereof away from the vertical alignment 25 2238 fi l ag g r'g igtf slzd z gg d ositiom a e o sai o u c c am ran 3 centrifuge as claimedin claim 7 wherein the surface of each of said bodies in engagement saidmeans for maintaining a substantially fixed vertitherewlth conslst of arnon'skld Surfacecal position comprises a ball swivel bearing cen-

1. A method of balancing a centrifuge of the type having a plurality ofseparate cups arranged around the periphery of a centrifuge wheel forthe purpose of holding containers of liquids to be centrifugedcomprising providing an annular chamber within the wheel, pivotallymounting the centrifuge at its axis as a pendulum, spring biasing thecentrifuge to a substantially vertical axis position, placing at leasttwo balancing weight bodies loosely within said annular chamber for freemovement therein, and then rotating the centrifuge at a speed in excessof that corresponding to the natural frequency of the spring biasedpendulum system to cause the balancing weight bodies to move within theannular chamber to positions compensating for any unbalanced conditionin the loading of the centrifuge cups, the balancing weight bodies beingpositioned within the annular chamber at substantially the same level asthe centrifuge cups during the rotation of the centrifuge.
 2. The methodas claimed in claim 1 wherein the centrifuge is mounted as an invertedpendulum.
 3. A method as claimed in claim 1 wherein the force of thesprings biasing the centrifuge to a substantially vertical axis positionis selected to provide a natural vibration frequency of the centrifugecorresponding to a centrifuge speed which is less than ten percent ofthe normal full operating speed of the centrifuge.
 4. A self-balancingcentrifuge comprising a rotatable shaft, means for supporting said shaftwhile permitting pivotal displacement thereof about a pivot point at thecenter line of said shaft, said supporting means including springbiasing means arranged to bias said shaft towards a substantiallyvertical position about said pivot point, a motor connected to rotatesaid shaft, a centrifuge wheel mounted upon said shaft at an axialposition thereon substantially displaced from said pivot point, saidcentrifuge wheel including a plurality of cups pivotally connected tothe periphery of said wheel for receiving sample containers of liquid tobe separated by the centrifuge action, said wheel defining an annularchamber arranged concentrically about said shaft, a counterbalance masspartially filling said annular chamber, said counterbalance mass beingarranged to shift in its angular position within said annular chamber toautomatically balance said wheel for unequal loading of said cups atspeeds above the natural vibration frequency of the combination of saidshaft and said wheel about said pivot point, said annular chamber beingformed to allow the shift of said counterbalance mass to a positionwhich is at substantially the same level as said cups during rotation ofsaid centrifuge wheel.
 5. A centrifuge as claimed in claim 4 whereinsaid counterbalance mass comprises a plurality of solid bodies.
 6. Acentrifuge as claimed in claim 5 wherein each of said solid bodies has acircular profile when viewed form above.
 7. A centrifuge as claimed inclaim 4 wherein said supporting means includes means for maintaining asubstantially fixed vertical position for said shaft while permittingrestrained pivotal deflections thereof away from the vertical alignmentposition.
 8. A centrifuge as claimed in claim 7 wherein said means formaintaining a substAntially fixed vertical position comprises a ballswivel bearing centered at said pivot point.
 9. A centrifuge as claimedin claim 4 wherein the combination of said shaft and said wheel issupported as an inverted pendulum with said pivot point positionedbeneath said wheel.
 10. A centrifuge as claimed in claim 4 wherein saidnatural vibration frequency corresponds to a centrifuge speed which isless than ten percent of the normal full operating speed of thecentrifuge.
 11. A centrifuge as claimed in claim 8 wherein saidsupporting means comprises a fixed supporting base mounting andsupporting said ball swivel bearing, a mounting plate axially spacedaway from said supporting base, said mounting plate being fixed withrespect to the axial position of said shaft, and said spring biasingmeans comprising compression springs arranged between said supportingbase member and said plate and spring biasing said plate to a positionsubstantially parallel to said base member.
 12. A centrifuge as claimedin claim 11 wherein said drive motor is mounted upon said mounting platewith its shaft extending vertically upward on an axis passing throughsaid pivot point, said rotatable shaft being connected to and forming anextension of the shaft of said drive motor and being supported upon anddirectly driven thereby.
 13. A centrifuge as claimed in claim 4, whereinsaid centrifuge wheel comprises a light weight removable sample trayincluding the peripheral portions of said wheel and the cups forreceiving sample containers and a portion defining said annular chamber,the sample tray portion of said wheel being removable from said annularchamber portion for the purpose of transferring the sample containers asa group for processing in another apparatus.
 14. A centrifuge as claimedin claim 4 wherein there is provided energy absorption means responsiveto displacement movements of said shaft away from the vertical positionto absorb the vibration energy of the combination of said shaft and saidwheel to thereby limit the vibration amplitudes at low frequencies. 15.A centrifuge as claimed in claim 14 wherein said energy absorption meanscomprises a plurality of hydraulic shock absorber devices.
 16. Acentrifuge as claimed in claim 4 wherein said spring biasing meansincludes snubber springs operable when the vibration excursion exceeds apredetermined amplitude to increase the spring rate and to thereby limitfurther vibration amplitude excursion to prevent damage to theapparatus.
 17. A centrifuge as claimed in claim 5 wherein theconfiguration of each of said solid bodies is such as to require slidingmovement upon the lower surface of said chamber for the shift inposition to achieve balance.
 18. A centrifuge as claimed in claim 17wherein at least one of said lower surface of said chamber and thesurface of each of said bodies in engagement therewith consist of anon-skid surface.